Lightning arrester

ABSTRACT

A voltage grading arrangement for lightning arresters particularly adapted for use in substations in which the conductors and the lightning arresters are enclosed within grounded metallic enclosures, the enclosures being filled with an &#39;&#39;&#39;&#39;electronegative&#39;&#39;&#39;&#39; gas such as sulphur hexafluoride (SF6) under pressure. The lightning arrester elements are enclosed in a porcelain housing and grading elements such as capacitors or resistors or both are disposed on the exterior of the porcelain housing and within the surrounding metal enclosure.

United States Patent 1 Kalb [111 3,733,521 1 May 15, 1973 [54] LIGHTNINGARRESTER [75] Inventor: John W. Kalb, Medina, Ohio [73] Assignee: TheOhio Brass Company, Mansfield, Ohio 22 Filed: Sept. 27, 1971 [21]Appl.No.: 183,812

[52] U.S. Cl ..3l7/70, 315/36 [51] Int. Cl. ..H02h 9/06 [58] Field ofSearch ..3l5/36; 317/70 [56] References Cited UNITED STATES PATENTS3,099,770 7/1963 Sorrow et al. ..315/36 2,298,114 10/1942 Estorff..317/70 3,414,759 12/1968 Connell et al. ..317/7() X PrimaryExaminer-James D. Trammell Attorney-Wm. Crighton Sessions et al.

[ 5 7] ABSTRACT A voltage grading arrangement for lightning arrestersparticularly adapted for use in substations in which the conductors andthe lightning arresters are enclosed within grounded metallicenclosures, the enclosures being filled with an electronegative gas suchas sulphur hexafluoride (SF under pressure. The lightning arresterelements are enclosed in a porcelain housing and grading elements suchas capacitors or resistors or both are disposed on the exterior of theporcelain housing and within the surrounding metal enclosure.

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BACKGROUND OF THE INVENTION This invention relates to lightningarresters and more particularly to voltage grading for lightningarrester assemblies for substations in which the conductors andlightning arresters are disposed in grounded metal enclosures that arefilled with a pressurized electronegative gas such as sulphurhexafluoride (SP although the invention is also useful in otherenvironments.

Present day lightning arresters embody a series of spark gaps utilizedwith either series connected valve blocks as in the Connell and PuttU.S. Pat. No. 3,414,759 or without series valve blocks as disclosed inmy co-pending application Ser. No. 1,186, filed Jan. 7, 1970. In orderto secure consistency and uniformity in sparkover voltage and todistribute the power line and surge voltages among the several gapsvarious types of voltage grading circuits are employed. By the use ofsuch circuits, the voltages across the arrester gaps can be distributedas desired among the gaps. The voltages would otherwise varysubstantially because in high voltage installations the arresters may bequite tall and the capacitance effects due to ground plane andsurrounding structures differ at different points along the lengths ofthe arresters.

Voltage grading, as is known in the art, may be arranged to distributethe voltage substantially evenly across the gaps or to arrange for moreof the voltage to be applied to certain of the gaps then others and thusto secure a cascading effect in sparkover and a lower sparkover voltage.As examples of patents illustrating grading systems see the Connell US.Pat. No. 3,496,409 and the Osterhus U.S. Pat. No. 3,377,503. In knownsystems, the components such as resistors, capacitors and auxiliary gapsthat make up the grading system are conventionally enclosed within anelongated tubular insulating housing, usually composed of porcelain,along with the other components of the arrester such as the gapelements, coils and valve blocks. The housing is gas-tight and theinterior of the housing is conventionally filled with nitrogen gas.

In conventional lightning arrester installations such known gradingschemes operate with a reasonable degree of effectiveness, althoughreliability and consistency of operation may be impaired and the voltagedistribution within the arrester changed by contamination of theexterior of the insulating housing or by other changes in theenvironment in which the arrester operates. Conventional voltagegrading, however, is not satisfactory in substations embodying gasinsulated systems (hereinafter sometimes referred to as ST systems,although other gases may be employed) yet such SF substations arebecoming of substantial economic importance in urban areas for thereason that SF substations occupy only a small fraction of the spacerequired by conventional substations and hence may be enclosed inbuildings or placed in urban locations where the cost of the land wouldotherwise make the installation of a substation prohibitively expensive.

The difficulty with lightning arresters in SE; substations arises fromthe fact that it is desirable to enclose the lightning arresters forsuch substations in metal enclosures which contain SF 8 gas underpressure, the enclosures being at ground potential. Since the enclosuresare located a comparatively small radial distance from the porcelainhousings of the arresters, (for example, in a typical installation theporcelain housings may have a diameter of 12 inches while the metalenclosures have a diameter of 30 inches) the internal components of thearresters, including the grading elements, are subjected to a very highradial field. The intemal nitrogen atmosphere within the porcelainhousing is severely overstressed by this high field and corona dischargewithin the housing to an unacceptable level results. The durability andreliability of the arresters and the consistency of sparkover voltagemay also be impaired.

SUMMARY A general object of the present invention therefore is toprovide lightning arresters embodying voltage grading that issubstantially unaffected by the external environment in which thelightning arrester is disposed. A more specific object is the provisionof a voltage grading system or arrangement for lightning arrestersparticularly adapted for use in connection with gas insulated systems inwhich the lightning arresters are enclosed in a housing filled with adielectric gas such as SP Further objects and advantages of theinvention will become apparent from the following description of thepreferred forms thereof.

Briefly, the above and other objects and advantages of the invention areattained by disposing the grading elements on the exterior of theporcelain housing for the arrester and, in the case of an arresterdesigned for service in gas insulated substations, within thepressurized and grounded enclosure surrounding the arrester proper. Withthis arrangement the internal elements of the arrester are substantiallyunaffected by ordinary environmental changes and are not subjected tothe extremely high radial voltage stresses to which they would otherwisebe subjected in SF systems. While in such installations the gradingelements on the exterior of the housing are subjected to a very intenseradial electrical field, this does not result in corona or otherelectrical disturbances because the elements are in an atmosphere ofpressurized SP or other similar electronegative gas which has theability to suppress corona.

The external grading can be used in conjunction with internal grading ofconventional nature if desired, or can be used without any internalgrading. For example, it is contemplated that standard commerciallightning arresters with the desired internal grading can be adapted foruse in gas insulated substations simply by enclosing them in porcelainhousings without weather sheds and disposing the grading components onthe exterior of the housings. The grading can be accomplished bycapacitors or resistors or combinations of capacitors and resistorsdisposed on the exterior of the porcelain housing. If the gradinginvolves resistance, the resistance can be provided by a conducting filmsuch as a film composed of a conductive glaze on the outside of theporcelain housing. It is preferable that longitudinally spaced,circumferentially extending conducting bands or other conductive meansbe provided along the exterior surface of the porcelain housing in orderto distribute electrical charges circumferentially about the outersurface of the housing.

BRIEF DESCRIPTION OF THE DRAWINGS view showing the present invention asadapted to a lightning arrester for service in a gas insulatedsubstation;

FIG. 2 is a diagrammatic view illustrating simple grading circuits thatmay be employed in the apparatus; and

FIG. 3 is a fragmentary diagram illustrating a modification of theexternal grading circuit.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring to FIG. 1 of thedrawings, a lightning arrester indicated in general at is shown inconjunction with an enclosed, gas insulated conductor 11 that leads to atransformer or other equipment that is to be protected from high voltagesurges resulting from lightning strokes, switching surges and the like.The conductor l1 conventionally is tubular and is supported within agas-tight, conductive enclosure in the form of a metal tube 12, theconductor being centered with re spect to the tube by insulators 13. Theconductor 11 is connected to the lightning arrester 10 by a conductor 14and the lightning arrester is disposed within a gastight enclosure 15that is attached in leakproof relation to the tube 12 by any convenientfitting 16. The enclosure 15, tube 12 and fitting 16 may all be made ofaluminum. The interior of the tube 12 and the interior of the enclosure15 are in open communication with each other, and like other elements ofthe enclosure for conductors in the substation, the tube 12 andenclosure 15 are filled with an appropriate electro-negative gas such asSP under pressure. The entire enclosing system, including the tube 12and the enclosure 15, is grounded. Various dielectric gases can beemployed such as halogenated hydrocarbons and sulphur hexafluoride.These gases are electro-negative. Their dielectric strength increasesgenerally (but not necessarily linearly) with pressure and they have theability to absorb large numbers of free electrons and to suppress coronadischarge to a much greater extent than nitrogen, the gas that isordinarily used in the housings of lightning arresters. The gas that isconsidered to be the most suitable at the present time is sulphurhexafluoride at a pressure of about 22 pounds per square inch gage.

The lightning arrester 10 may take any conventional or appropriate formfor the duty to which the arrester is subjected. In FIG. 1 the internalarrester components indicated in general at 20 are made in accordancewith my aforesaid application Ser. No. 1,186, to which reference is madefor a complete description. Briefly, the arrester elements includeparallel-connected coil and non-linear resistor assemblies 21 and packs22 of gap units in which the spark gaps themselves are disposed. The gapunits provide serially connected spark gaps and the assemblies 21 arealso connected in series with the spark gaps. Conventional gradingcomponents may or may not be included with the gap units depending onthe design and service requirements. The number of assemblies 21 and gappacks 22 depends upon the system voltage and the design of the arresterelements but, as an example, for a 345 kV substation about 15 gap packsand associated coil assemblies in a stack about 10 feet high would berequired to provide a reasonable sparkover voltage of about 540 kV forsuch a system.

The arrester components are enclosed within a gastight tubularinsulating member such as the porcelain housing 23. It is to be notedthat the housing 23 is not provided with weather sheds, these beingunnecessary since the arrester is protected from the environment by theenclosure 15. The housing is closed by conductive end plates such ascast aluminum plates 24 and 25 which are secured in sealed relationshipby conventional seals to the ends of the housing 23 and the top andbottom coil assemblies 21 are electrically connected to the end plates24 and 25 respectively by conventional means. The interior of thecompleted enclosure is filled with dry nitrogen gas at normalatmosphefic pressure in accordance with known practices. It is notdesirable to use SF in the interior of the arrester housings because thedielectric strength of SP is too high to permit proper operation ofconventional gaps at the desired sparkover voltage. The top plate 24 isconnected to the conductor 11 by the conductor 14; the entire assemblyis disposed within the metal enclosure 15 with the bottom plate 25supported by an aluminum spacer 26 that rests on the bottom 27 of theenclosure and centers the arrester with respect to the enclosure. Thebottom plate 25 is electrically connected to the enclosure 15 throughthe spacer 26 and the enclosure 15 is grounded in any conventionalmanner as well as being connected at its upper end to the groundedtubular enclosure 12.

With this construction it will be evident that with any reasonablerelationship between the diameter of the porcelain housing 23 and themetal enclosure 15, which is at ground potential, there will be a veryhigh radial field between the internal parts of the arrester that are athigh potential and'the grounded enclosure 15. Under such conditions thenitrogen gas within the housing 23 would be overstressed and there wouldbe severe corona problems. The result would be that the reliability andconsistency of operation of the arrester would be seriously impaired andunacceptable levels of radio noise might be generated.

According to the present invention these difficulties are eliminated bydisposing grading elements on the exterior of the porcelain tube 23 sothat the environment that the internal parts of the arrester see" is apotential that gradually decreases from the line voltage at the top ofthe arrester to ground voltage adjacent the bottom of the arrester. Inthe present embodiment the grading is accomplished by capacitorsdisposed on the exterior of the porcelain housing 23. A series ofspaced, preferably annular conductive bands 28 are disposed on theexterior of the porcelain housing 23. The bands are thin and may be madeof copper foil adhesively secured to the exterior of the housing. Thebands furnish convenient connections for the grading elements such asthe capacitors 29, and also distribute the voltage circumferentiallyaround the housing. Adjacent bands are connected by capacitors 29. Theuppermost band 28 is connected to the member 24 as by a coating 30 ofconductive paint, and the lowermost band is connected to the member 25by a similar coating 31 of conductive paint. Instead of capacitors,resistors or combinations of resistors and capacitors can be employed ifdesired. The effect is to distribute the voltage between line and groundthroughout the length of the housing 23 and thus the internal parts ofthe arrester are not subject to undue radial stresses. While thecomponents on the exterior of the porcelain housing are subjected to avery strong radial field, this does not create any problem in thepressurized SF atmosphere within the enclosure since that gas has thecapability of suppressing corona and has high dielectric strength.

If it is desired to provide substantially uniform voltage distributionalong the housing, greater capacitance is required near the top ofhousing 23 than near the bottom as indicated by the progressivelydecreasing number of capacitors 29 shown in FIG. 1 from the top to thebottom of the housing. The following table gives an example ofappropriate values of capacitance for an arrester as described abovewith the arrester elements enclosed in a porcelain housing 23 aboutfeet, six inches tall, having a diameter of about 12 inches within ametal enclosure having a diameter of 30 inches, the enclosure beingfilled with SF at a pressure of 22 p.s.i.g. The exterior of the housing23 is divided into twenty segments by the bands 28. For each segment,the capacitance to the grounded enclosure 15 is about 12 pico farads(pf). The table indicates the capacitance, calculated to the nearest 5Opf, required to give substantially uniform voltage distribution alongthe housing. In the table the capacitance values are given for thecapacitors adjacent each segment of the housing starting with the top orline potential end and progressing to the bottom of ground end of thehousing, Segment No. l being between the first and second bands 28,Segment No. 2 being between the second and third bands 28, and so on.

Segment No. Capacitance Segment No. Capacitance (P (PO 1 2300 l l 700 22 100 12 600 3 I900 13 500 4 1700 14 400 5 1550 15 300 6 1300 16 250 7 ll 50 l 7 200 8 1000 l 8 200 9 900 I9 150 10 800 150 As noted above, thisarrangement can be used either with or without internal grading and ifinternal grading is employed the grading can be arranged to cause onegap or group of gaps in the arrester to fire first in manners known tothose skilled in the art. The external grading can be arranged toprovide substantially equal voltage drops between successive bands 28or, if desired, the external grading can be arranged to cause greatervoltage to be imposed upon one gap or group of gaps thereby causing thatgap or group of gaps to fire first upon subjecting the arrester tovoltage in excess of its spark-over voltage.

The arrangement has important advantages from an operational standpointsince the environment in which the arrester operates in effect remainsconstant. There is no chance of contamination of the exterior of theporcelain housing and contamination of the exterior of the enclosure 15does not affect the operation of the arrester since this enclosure is atground potential at all times. Because of the constant environment inwhich the arrester elements operate, consistency of operation andreliability in service are readily obtainable.

FIG. 2 is an electrical diagram showing a simple internal grading schemeused in conjunction with the arrester elements and external grading ofFIG. 1. In this embodiment of the invention the conductor 11 and itstubular housing 12, the enclosure 15, the porcelain housing 23 and thearrester components are the same as in FIG. 1 and the same referencecharacters have been applied to corresponding parts. The arrestercomponents are shown diagrammatically. Each assembly 21 is made up of acoil 35 and a non-linear resistor 36 in the form of a valve block andeach gap pack 22 comprises a plurality of gaps 37.

The internal grading is accomplished by parallelconnected capacitors 39and resistors 40 connected across the gaps 37. The grading on theexterior of the housing is provided by capacitors 29 as previouslydescribed.

Other intemal grading schemes may be employed such as that shown in theaforesaid Connell US. Pat. No. 3,496,409. Also, as shown in FIG. 3, theexternal grading may include not only capacitance as provided bycapacitors 29, for example, but also resistance as diagrammaticallyindicated at 41.

From the foregoing it will be evident that the invention provides animproved grading system for lightning arresters that is particularlyadapted for use in gasinsulated substations. The external gradingemployed in the present system insures that the internal elements of thelightning arresters are substantially unaffected by ordinary changes inthe environment and thus consistency and reliability of operation of thearresters is improved.

I claim:

I. A lightning arrester assembly for a gas-insulated electrical systemin which the conductors are enclosed within grounded metallic enclosingmembers containing gas, said assembly embodying a lightning arrestercomprising a stack of gap elements providing a plurality of seriallyconnected spark gaps, an elongated sealed housing composed of insulatingmaterial surrounding said gap elements, said housing having atone endthereof a first conductive member that is at line potential when thearrester is in service and a grounded conductive member at the other endthereof, a grounded conductive enclosure surrounding and spaced fromsaid tubular housing, an electro-negative dielectric gas filling thespace between the exterior of said tubular housing and the interior ofsaid conductive enclosure and voltage grading means on the exterior ofsaid insulating housing extending between said conductive members anddisposed within said conductive enclosure and the gas therein.

2. A lightning arrester assembly according to claim 1 wherein the gaswithin said conductive enclosure is under super atmospheric pressure.

3. A lightning arrester assembly according to claim 2 wherein said gasis sulphur hexafluoride.

4. A lightning arrester assembly according to claim 1 having connectingmeans providing open communication between the interior of the enclosureand the interior of one of said enclosing members.

5. A lightning arrester assembly according to claim 4 in which saidconnecting means also constitutes an electrical connection between saidenclosure and said one of said enclosing members.

6. A lightning arrester assembly according to claim 5 having anelectrical connection extending through said connecting means from oneof said conductors to said first conductive member.

7. A lightning arrester assembly according to claim 1 in which saidinsulating housing is a tubular porcelain member.

8. A lightning arrester assembly according to claim 1 in which thegrading means on the exterior of the housing comprises a series ofannular conductive bands spaced along the exterior of the insulatinghousing and grading elements interconnecting at least some of saidbands.

9. A lightning arrester assembly according to claim 8 wherein thegrading elements include capacitors disposed on the exterior of thehousing.

10. A lighting arresting assembly according to claim 8 wherein thegrading elements comprise capacitors and resistance elements disposed onthe exterior of the housing.

11. A lightning arrester assembly accordingto claim lating housingbetween said conductive members.

13. A lightning arrester according to claim 12 in which said insulatinghousing is a tubular porcelain member.

14. A lightning arrester according to claim 12 in which the gradingmeans comprises a series of annular conductive bands spaced along theexterior of the insulating housing and grading elements interconnectingat least some of said bands.

15. A lightning arrester according to claim 14 wherein the gradingelements include capacitors disposed on the exterior of the housing.

16. A lightning arrester according to claim 14 wherein the gradingelements comprise capacitors and resistance elements disposed on theexterior of the housing.

17. A lightning arrester according to claim 12 also having grading meanswithin said housing.

1. A lightning arrester assembly for a gas-insulated electrical systemin which the conductors are enclosed within grounded metallic enclosingmembers containing gas, said assembly embodying a lightning arrestercomprising a stack of gap elements providing a plurality of seriallyconnected spark gaps, an elongated sealed housing composed of insulatingmaterial surroUnding said gap elements, said housing having at one endthereof a first conductive member that is at line potential when thearrester is in service and a grounded conductive member at the other endthereof, a grounded conductive enclosure surrounding and spaced fromsaid tubular housing, an electronegative dielectric gas filling thespace between the exterior of said tubular housing and the interior ofsaid conductive enclosure and voltage grading means on the exterior ofsaid insulating housing extending between said conductive members anddisposed within said conductive enclosure and the gas therein.
 2. Alightning arrester assembly according to claim 1 wherein the gas withinsaid conductive enclosure is under super atmospheric pressure.
 3. Alightning arrester assembly according to claim 2 wherein said gas issulphur hexafluoride.
 4. A lightning arrester assembly according toclaim 1 having connecting means providing open communication between theinterior of the enclosure and the interior of one of said enclosingmembers.
 5. A lightning arrester assembly according to claim 4 in whichsaid connecting means also constitutes an electrical connection betweensaid enclosure and said one of said enclosing members.
 6. A lightningarrester assembly according to claim 5 having an electrical connectionextending through said connecting means from one of said conductors tosaid first conductive member.
 7. A lightning arrester assembly accordingto claim 1 in which said insulating housing is a tubular porcelainmember.
 8. A lightning arrester assembly according to claim 1 in whichthe grading means on the exterior of the housing comprises a series ofannular conductive bands spaced along the exterior of the insulatinghousing and grading elements interconnecting at least some of saidbands.
 9. A lightning arrester assembly according to claim 8 wherein thegrading elements include capacitors disposed on the exterior of thehousing.
 10. A lighting arresting assembly according to claim 8 whereinthe grading elements comprise capacitors and resistance elementsdisposed on the exterior of the housing.
 11. A lightning arresterassembly according to claim 1 also having grading means within saidhousing.
 12. A lightning arrester comprising a plurality of seriallyconnected spark gaps, a sealed elongated housing composed of insulatingmaterial surrounding said spark gaps, a conductive member that is atline potential when the arrester is in service in contact with one endof said housing, a conductive member that is at ground potential incontact with the other end of said housing, and voltage grading means onthe exterior of said insulating housing between said conductive members.13. A lightning arrester according to claim 12 in which said insulatinghousing is a tubular porcelain member.
 14. A lightning arresteraccording to claim 12 in which the grading means comprises a series ofannular conductive bands spaced along the exterior of the insulatinghousing and grading elements interconnecting at least some of saidbands.
 15. A lightning arrester according to claim 14 wherein thegrading elements include capacitors disposed on the exterior of thehousing.
 16. A lightning arrester according to claim 14 wherein thegrading elements comprise capacitors and resistance elements disposed onthe exterior of the housing.
 17. A lightning arrester according to claim12 also having grading means within said housing.